1. Field of the Invention
The present invention relates to a method for manufacturing a tyre. More particularly, the present invention relates to a method for manufacturing a tyre wherein a green tyre is assembled onto a rigid toroidal support, and then molded and cured in a vulcanization mold defining a molding cavity, which includes at least a portion in which molding and curing are performed at a constant volume.
2. Description of the Related Art
Generally, in a tyre production cycle, after a manufacturing process in which the different tyre components are formed and assembled to provide a green tyre (i.e. an uncured or raw tyre), a molding and curing process is carried out for the purpose of stabilizing the tyre structure to a given geometric conformation, generally characterized by a particular tread pattern. To this aim, the tyre is introduced into a vulcanization mold, typically comprising a pair of side parts adapted to be axially moved close to each other, which are arranged to operate on the tyre bead and sidewalls, and at least one series of circumferentially distributed sectors adapted to be radially moved close to each other, so as to operate on the tyre tread band. In more detail, side parts and sectors are mutually movable between an open condition, in which they are spaced apart from each other to enable loading of the tyre being processed, and a closed condition, in which they define a molding cavity, the inner surface of which is the same as the outer surface of the tyre to be obtained.
In a known method of molding and curing a tyre, a rigid toroidal support having the same configuration as the inner surface of the tyre to be obtained is arranged within the tyre. For example, U.S. Pat. No. 4,895,692 discloses a mold for molding and curing an elastomeric tyre comprising a rigid core defining an interior surface of the tyre, two side parts and a peripheral ring divided into a plurality of segments. The members of the mold assure the molding of the exterior and interior surfaces of the tyre and completely define a molding space for the tyre. The vulcanization is therefore effected with constant volume.
A different approach is disclosed in PCT patent application WO 01/00395, that discloses a method in which a green tyre manufactured on a toroidal support is closed in a vulcanization mold. The side portions of the tyre are sandwiched between the mold side parts and the toroidal support. Steam or another fluid under pressure is fed to a diffusion interspace formed, due to tyre expansion, between the inner surface of the tyre and the outer surface of the toroidal support. More particularly, following closure of the mold, the tyre is enclosed in a holding space confined between the outer surface of the toroidal support and the inner walls of the molding cavity. The holding space when the mold is closed has a volume greater than the volume taken up by the tyre itself. In more detail, the holding space has two radially inner portions of shape and size substantially corresponding to the shape and size of the side portions of the tyre, and a radially outer portion confined between said radially inner portions, of radial dimensions greater than the radial dimensions, i.e. thickness, measured on the radially outer portion of the tyre itself
Recently new methods for manufacturing tyres have been proposed which do not need separate manufacturing of semi-finished products used in the traditional methods, for example the tread band, the sidewall strips, the carcass plies, the belt strips and the bead cores, just to mention the main ones. According to the above innovative processes, those semi-finished products are replaced by a few number of basic components which are manufactured “in situ” when the green tyre is assembled. These basic components are elongated elements, usually in the form of a strip or of a ribbon, made from raw elastomeric material, possibly reinforced with one or more reinforcing cords.
This new process for the assembly of the tyre is carried out by means of deposition, on the abovementioned toroidal support, of the above basic components designed to form the structural parts of the tyre (substantially corresponding to the parts made with the usual semi-finished products according to conventional manufacturing methods), using few types of movement, such as radial deposition, directed towards the axis of rotation of the toroidal support, and circumferential deposition onto the surface of the toroidal support made to rotate about its axis, or a combination of the two.
The basic components are supplied to said toroidal support in the form of continuous elongated elements. The radially deposited components may be cut beforehand into portions of predefined size, while the circumferentially deposited components are cut after winding onto the drum. These basic components have typically a cross section with dimensions smaller than those of the structural part to be constructed.
Once the assembly steps are concluded, the green tyre prepared with the above described method is closed in a vulcanization mold, for instance of the type disclosed in the above cited patent application WO 01/00395, and eventually cured.